1. Field of the Invention
The present invention relates to an image reader, and particularly to the improvement of image readers of the type in which the image of an original text is formed on a linear image sensor by means of an optical imaging system.
2. Description of the Prior Art
Conventional image readers of this type are designed to read an original text placed on a platen by projecting a slit-shaped light beam produced by an illumination light source onto the entire text surface continuously and focusing the reflected slit-shaped light beam from the text surface on a linear image sensor by means of an optical imaging system. However, the text image formed on the linear image sensor generally decays (curvature of field) due to the presence of the optical imaging system, and therefore the image signal produced by the image sensor also decays.
This problem has been overcome conventionally through the high-band emphasizing process called MTF (Modulation Transfer Function) correction process for the crude image signal produced by the image sensor so that the decay of image is corrected (refer to publication entitled "Digital Image Processing", written by Makoto Nagao, published by Kindai Kagaku Corp. on Dec. 10, 1978, for example).
However, the decayed image correction scheme of the conventional image sensor is intended to act on the image sensor output invariably, instead of treating image information on a case by case basis, and therefore it is not yet satisfactory for the delivery of high quality image information.
Specifically, the optical imaging system of the conventional image reader has an imaging performance (MTF) which varies depending on the direction of field angle of the focus lens (parallel to the scanning direction of the image sensor), as shown in FIG. 12. Therefore, a constant decay correction process does not clear the image signal of the focus error distributed along the field angle direction of the focus lens, and the system fails to provide high quality image information.
Particularly, in a color image reader, the focus lens has different imaging performances (MTF) for separate color components, i.e., red (R), green (G) and blue (B), as shown in FIG. 13, and therefore even after the constant image decay correction process for the image signal, the focus error which is dependent on the color component is left undissolved in addition to the focus error which is dependent on the field angle of the focus lens. Accordingly, conventional color image readers involve the serious technical subject of drawing high quality image information from original texts.